We tend to think of Attention-Deficit/Hyperactivity Disorder (ADHD) as a condition for kids. But adult ADHD is a thing—and new research suggests that it manifests itself differently than the childhood diagnosis.

Two new large studies published this month in JAMA Psychiatry say this could explain why up to five percent of the adult population is diagnosed with adult ADHD, even without having a diagnosis as a child.

According to the Centers for Disease Control and Prevention (CDC), ADHD is one of the most common neurodevelopmental disorders of childhood. It’s usually first diagnosed when someone is a kid and may last into adulthood, although it’s commonly thought that people outgrow it. Children with ADHD typically have trouble paying attention, difficulty controlling their impulses, or are overly active.

But people who suffer from adult ADHD have markedly different symptoms—namely, they may have unstable relationships or poor performance at work.

“The primary difficulty for an adult with ADHD is this: They don’t keep agreements,” Michael Manos, Ph.D., director of the Center for Pediatric Behavioral Health at the Cleveland Clinic, who also works with adult ADHD patients, tells SELF. This can cause issues in a person’s life, from relationship problems (repeatedly saying you’ll do something that you never do) to anxiety and depression, often due to constantly having a to-do list hanging over you head that never gets done, he says.

Jessica Agnew-Blais, Sc.D., a postdoctoral researcher at King’s College London and lead author of one of the new studies, tells SELF that she was “a bit surprised” by her findings that nearly 70 percent of people with adult ADHD didn’t meet the criteria for the disorder based on reports from their moms and teachers that were collected when the participants were five, seven, 10, and 12 years old. “While we knew that ADHD could persist from childhood to adulthood, it has been widely believed that ADHD has its origins in childhood, and adult ADHD has been generally understood as a continuation of childhood ADHD,” she says.

The second study, which was conducted in Brazil by separate researchers, followed more than 5,000 children, who were assessed for ADHD at age 11 and again at 18 or 19. In their late teens, nearly 12 percent had ADHD but, among those, only 12 percent had received a childhood diagnosis.

Agnew-Blais has a few possible explanations for the findings: One, adult ADHD could be a distinctly different disorder from childhood ADHD. Two, other mental health problems, such as anxiety, can mimic the symptoms of ADHD, and it’s possible that a proper diagnosis was never made. And three, people with late-onset ADHD could have had ADHD in childhood but it was never picked up.

Manos supports the latter explanation. “Many kids who would be considered ‘sub-threshold’ to ADHD when they’re younger can get by,” he says. “If you have a person who is quite bright and meets the demands of school quite well, ADHD doesn’t show up as a problem. I can’t tell you how many adults I diagnose with ADHD who say school was a breeze.”

But, when someone becomes an adult and has different responsibilities, such as children and a demanding job that requires constant attention, the symptoms can manifest themselves.

Adult ADHD is treated the same as it is in childhood, Manos says—through stimulant medications, such as Ritalin and Adderall, and behavioral intervention. If an adult with ADHD is suffering from anxiety or depression as well, counseling can also be beneficial, he says.

If you suspect you might have adult ADHD, experts say it’s important to speak with a psychologist or psychiatrist to get evaluated—and don’t be ashamed of it. “It’s important to know that adult ADHD is relatively common, so you’re not alone,” Agnew-Blais says.